I reckon this is the best contract deal for the Samsung Galaxy S7 edge for a reason monthly cost and <£100 upfront cost with decent 3Gb data on O2 - all in for £696.
I have linked to the black handset but seems to be available on all colours.
Use ELF15 instead to get £15 off handset cost, otherwise £75 upfront.
Note this tariff was previously £125 upfront, I reckon its been updated very recently.
Possible to get this even cheaper via £40 topcashback
making this £671 all in - they will decline it if you use the code.
UPDATE 29/11/16: updated working £15 discount code ELF15